1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive employing a sync mark detector comprising a matched filter and a dual polarity correlator.
2. Description of the Prior Art
FIG. 1 shows a prior art format of a disk 2 employed in a disk drive of a computer system. The disk 2 comprises a plurality of radially-spaced, concentric tracks 4 which are partitioned into a plurality of data sectors. The disk 2 further comprises a plurality of embedded servo sectors 6 for use in servoing a head over the desired track during write and read operations. A sector (data or servo) typically comprises a preamble field 8 for synchronizing timing recovery and gain control within a read channel, and a sync mark 10 for symbol synchronizing to a data field 12. Data stored in an embedded servo sector 6 may include a track address which provides coarse head positioning information to a servo control system. An embedded servo sector 6 also typically comprises a plurality of servo bursts 14 recorded at precise offsets from a track""s centerline to provide fine head positioning information to the servo control system.
It is desirable to detect the sync mark 10 reliably to minimize the number of retries as well as the number of unrecoverable sectors. Prior art disk drives typically employ a data detector for detecting an estimated data sequence from the read signal, and a correlator for correlating the estimated data sequence with the sync mark pattern. The sync mark 10 is detected when the output of the correlator exceeds a predetermined threshold. However, the prior art techniques for detecting the sync mark 10 are still susceptible to errors, such as missing or falsely detecting the sync mark 10 due to noise in the read signal.
There is, therefore, a need to improve the reliability of the sync mark detection process within a disk drive to minimize the number of retries as well as the number of unrecoverable sectors.
The present invention may be regarded as a disk drive comprising a disk having a plurality of data tracks, each data track comprising a plurality of sectors. Each sector comprises a preamble field for storing a predetermined periodic data sequence, a sync field for storing a predetermined sync mark pattern, and a data field for storing data. A head is actuated radially over the disk for generating a read signal. A sync mark detector detects the sync mark pattern in the read signal, wherein the sync mark pattern is recorded on the disk in a binary sequence. A xe2x80x9c1xe2x80x9d bit in the binary sequence is represented by a dibit recorded on the disk, the dibit generating a dibit response in the read signal, and a xe2x80x9c0xe2x80x9d bit in the binary sequence is represented by the absence of a dibit recorded on the disk. The sync mark detector comprises a matched filter having an impulse response substantially matched to the dibit response, the matched filter for filtering the read signal to generate a filtered signal. A first threshold comparator compares the filtered signal to a positive threshold and to a negative threshold, wherein the first threshold detector outputs a first estimated sequence corresponding to the positive threshold and a second estimated sequence corresponding to the negative threshold. A xe2x80x9c1xe2x80x9d bit is detected in the first estimated sequence when the filtered signal exceeds the positive threshold and a xe2x80x9c0xe2x80x9d bit is detected in the first estimated sequence when the filtered signal does not exceed the positive threshold. A xe2x80x9c1xe2x80x9d bit is detected in the second estimated sequence when the filtered signal exceeds the negative threshold and a xe2x80x9c0xe2x80x9d bit is detected in the second estimated sequence when the filtered signal does not exceed the negative threshold. A first correlator correlates the first estimated sequence with the predetermined sync mark pattern to generate a first correlated signal, and a second correlator correlates the second estimated sequence with the predetermined sync mark pattern to generate a second correlated signal. A second threshold comparator compares the first and second correlated signals to a threshold, wherein the sync mark pattern in the read signal is detected when either the first or second correlated signals exceeds the threshold.
In one embodiment the read signal comprises a plurality of polarity alternating pulses, the sync mark detector generates a polarity signal relative to which correlated signal crosses the threshold, and the polarity signal is indicative of an expected polarity of the pulses in the read signal representing the data stored in the data field.
The present invention may also be regarded as a method of detecting a sync mark pattern in a disk drive, the disk drive comprising a disk comprising a plurality of data tracks, each data track comprising a plurality of sectors, each sector comprising a preamble field for storing a predetermined periodic data sequence, a sync field for storing the sync mark pattern, and a data field for storing the data, wherein the sync mark pattern is recorded on the disk in a binary sequence. A xe2x80x9c1xe2x80x9d bit in the binary sequence is represented by a dibit recorded on the disk, the dibit generating a dibit response in the read signal, and a xe2x80x9c0xe2x80x9d bit in the binary sequence is represented by the absence of a dibit recorded on the disk. A head is actuated radially over the disk to generate a read signal. The read signal is filtered with a matched filter having an impulse response substantially matched to the dibit response to generate a filtered signal. The filtered signal is compared to a positive threshold and to a negative threshold to generate a first estimated sequence corresponding to the positive threshold and a second estimated sequence corresponding to the negative threshold. A xe2x80x9c1xe2x80x9d bit is detected in the first estimated sequence when the filtered signal exceeds the positive threshold and a xe2x80x9c0xe2x80x9d bit is detected in the first estimated sequence when the filtered signal does not exceed the positive threshold. A xe2x80x9c1xe2x80x9d bit is detected in the second estimated sequence when the filtered signal exceeds the negative threshold and a xe2x80x9c0xe2x80x9d bit is detected in the second estimated sequence when the filtered signal does not exceed the negative threshold. The first estimated sequence is correlated with the sync mark pattern to generate a first correlated signal, and the second estimated sequence is correlated with the sync mark pattern to generate a second correlated signal. The first and second correlated signals are compared to a threshold, wherein the sync mark pattern is detected in the read signal when either the first or second correlated signals exceeds the threshold.